The major objective of these studies is to understand how the metabolism of cyclic GMP is regulated. Through such an understanding, we hope to gain better insight into the role of cyclic GMP and into mechanisms involved in the actions of some hormones and neurotransmitters. Our objective will be approached by studying cyclic GMP levels in intact cells and by studying the regulation of guanylate cyclase and phosphodiesterase activities in cellfree systems and intact cells. Specifically, the following aims will be pursued: 1) To explore the mechanisms(s) whereby agents raise cyclic GMP levels in intact cells through a calcium-dependent process. 2) To study the regulation of membrane-associated guanylate cyclase in mammalian tissues and to compare its responsiveness to potential modifiers with that of the soluble form of the enzyme. 3) To compare the effects of fatty acids and other lipids on the activities of crude, partially purified and homogeneously purified soluble guanylate cyclase. 4) To assess the importance of the calcium-dependent regulator protein in controlling cyclic GMP phosphodiesterase activity in intact cells and to examine the effects of endogenous agents on the interaction of the protein with phosphodiesterase. 5) To assess the extent to which the number and properties of major multiple forms of phosphodiesterase depend upon assay conditions, proteolysis and other factors in an attempt to answer the question: Are major multiple forms of the enzyme real or artifactual?